The present invention relates to an improved process and apparatus for producing a useful gas from solid fuels. Specifically, the invention relates to a novel process and apparatus for producing a medium BTU clean gas from a solid fuel, such as coal, without manufactured oxygen. Moreover, the process and apparatus provides for the production of other products having direct commercial utility with virtually no solid or liquid waste.
Coal is the world's most abundant fuel resource. However, coal has limited commercial applications as an energy source due to its many practical limitations such as difficulties of transport and incompatibility with power generating devices.
Coal gasification processes have been developed which attempt to transform the coal from a carbonaceous solid fuel to a gas fuel which has much more practical utility. Such a system, for example, was disclosed in my U.S. Pat. No. 4,555,249 for "Process for Gasification of coal and Organic Solid Wastes" and U.S. Pat. No. 4,274,839 for "Process for Gasification of Coal and Organic Wastes" which are hereby incorporated-by-reference. Generally, gasification processes provide a means for converting combustible organic materials such as coal, wood, tar sand, shale oil, and municipal, agriculture or industrial waste into a gas end product typically consisting of hydrogen or methane gas. The gas end product is then commonly utilized in a downstream phase of the process. For example, the gas product may be used to produce steam for the production of electricity or heating by passing the hot gases through a steam generation zone. Moreover, the production gases are often utilized in a downstream chemical process for further production. If the gas which is produced is a high grade gaseous stream it may be recovered for direct commercial use as a fuel energy source.
In order to produce a gaseous end product which has direct commercial utility--for example to drive a gas turbine or as a clean compression fuel source for use in an automobile engine--the gas end product must have a useful BTU level or grade. In this, a clean high BTU grade gas (viz. approximately 300 BTU/C.F.) is most preferable. However, a clean medium BTU grade (viz. approximately 200-275 BTU/C.F.) gas is also sufficient as a energy source for direct commercial uses. Significantly, however, a low BTU gas (viz. approximately 125-175 BTU/C.F.) is not a useful gaseous product in direct commercial applications. Moreover, a gas product of any grade is not acceptable if it contains contaminates which adversely affect its combustion properties. For example, a gas end product which contains large amounts of carbon dioxide, nitrogen, and sulfur compounds such as hydrogen sulfide ("sulfur gas") can not be used as a direct energy source for commercial applications. A gas end product having large amounts of contaminates is not acceptable in direct commercial applications, for example in gas turbines, because it will produce flame-out and stoppage. Moreover, the combustion by-products of a contaminated gas will produce environmentally unsafe by-products (e.g. SO.sub.x gas, NO.sub.x gas, particulate, etc.) which are unacceptable in commercial applications.
Common to all gasification techniques is the need for in process oxygen to carry out the necessary reactions (viz. to react with the carbon of the carbonaceous solid fuel). As a general matter, if the source of in-process oxygen for the gasification process is derived from manufactured oxygen, then the gaseous end product will be a high grade BTU gas. Conversely, if the source of in-process oxygen for the gasification process is derived from air or steam, then the gaseous end product will be a low grade BTU gas. For example, certain gasification processes currently employed by Texaco, Dow, and Shell require a high amount of in-process oxygen in order to produce a useful BTU gas end product. Although the gas end products of the Texaco, Dow, and Shell processes are high grade BTU gases (viz. approximately 300 BTU/C.F.), the processes require the use of manufactured oxygen. The gasification process currently employed by British Gas uses air and steam as a source of in-process oxygen, but the gas end product is a low grade BTU gas having limited commercial utility. The problem with utilizing manufactured oxygen as the source of in-process oxygen is that it has a commercially prohibitive cost. Manufactured oxygen can be one of the most significant costs in a gasification process. Manufactured oxygen is typically produced through a cryogenic method wherein a volume of air is reduced to extremely low temperatures--in the order of 360.degree. F. below zero--whereby the O.sub.2 is liquidized and removed in a pure liquid form. Current market rates for manufactured oxygen are approximately from five times the cost of on-site coal. Moreover, an oxygen gasification process requires about one ton of oxygen for every ton of coal. Accordingly, the high cost of manufactured oxygen adversely affects the economic efficiency of a gasification process. A common denominator of all gasification systems is that they must be economical to operate. Gasification systems have large initial capital investment cost and as a result, a low process efficiency is unacceptable to gasification management teams. In this, coal deposit owners are discouraged form using gasification techniques which are capable of producing a high BTU gas for direct commercial applications--for example to drive a gas turbine or as a clean compression fuel source for use in an automobile engine--which prevents the expansion of use of clean energy sources by the public. For example, a ready and cost effective source of compressed hydrogen would encourage automobile manufacturers to develop some hydrogen fueled automobiles. As noted, in order to effectively operate gas turbines, a medium to high grade BTU gas is required. Electrical power producers are discouraged by the high cost of coal gasification and have in the past almost exclusively utilized natural gas sources.
Manufactured oxygen has been the preferable source of process oxygen because it not only provides the necessary reaction content for the creation of a high grade BTU gas, but an excess amount of non-reacted O.sub.2 is burned in order to create additional and necessary process heat. In order to eliminate the need for manufactured oxygen attempts have been made to use a process catalyst which accelerates the process reactions in order to provide beneficial temperature affects. One such attempt has been made by Exxon Research and Engineering Co. wherein the process reactions are carried out in the presence of a carbon-alkali metal catalyst. However, this Exxon gasification process and reaction catalyst have proven ineffective and problematic. The carbon-alkali metal catalyst of the prior art consist of an alkali metal (e.g. Na) with impregnated carbon. The alkali and carbon, however, are not chemically bonded, but merely coexist in their respective forms. Typically, the catalyst, in liquid form, is sprayed onto a fine coal and delivered to a reaction vessel. The inherent problem with the prior art catalyst is that once the gasification reactions are complete, the catalyst must be separated from the reaction products, such as coal ash, for disposal and/or recycling. This separation step involves the use of complex reactor designs and additional hardware which not only increase the complexity of the system, but increase capital cost significantly. As such, the prior art gasification systems which utilize reaction catalysts have proven to be commercially unacceptable. The prior art gasification systems have failed to provide a recycle reagent which serve as a process catalyst while providing superior recycling and density properties.
Prior art gasification methods and systems have proven disadvantageous for several other significant reasons. First, the prior art thermal gasification systems and methods require very high operating temperatures--approximately 2500.degree. to 2800.degree. F.--in order for the process reactions to occur. At these extreme operating temperatures, the iron based reaction vessels will melt if cooling mechanisms are not in place. Typically, such mechanism include complex and expensive vessel insulation schemes and/or heat exchanger cooling. Moreover, the high reaction temperatures require the use of expensive iron alloys--such as 310 (Cr/Ni) S.S. as fabrication material for the reaction vessels.
The prior art methods and systems have relatively low thermal process efficiencies. The prior art methods and systems have been unable to maximize the extent of gasification which occurs during the process thereby obtaining relatively low conversion efficiencies. The prior art methods and systems produce environmentally unsafe by-product waste which requires costly post process handling. The prior art techniques have been unable to produce commercially useful by-products from the gasification process. The prior art gasification methods and systems require the manufacturing of special process modules and hardware which increase production cost and make it more difficult to relocate from one mine site to another as resources change. The prior art systems which utilize manufactured oxygen in the production of high grade BTU gas, produce a product gas having very high exit temperatures which can not be directly used in gas turbines.
The difficulties and limitations suggested in the preceding are not intended to be exhaustive, but rather are among many which demonstrate that although significant attention has been devoted to solid gasification methods and systems, such methods and systems appearing in the past will admit to worthwhile improvement.